An OLED display panel is generally comprised of an array of organic light emitting diodes (OLEDs) that have carbon-based films or other organic material films between two charged electrodes, generally a metallic cathode and a transparent anode typically being glass. Generally, the organic material films are comprised of a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When voltage is applied to the OLED cell, the injected positive and negative charges recombine in the emissive layer and create electro-luminescent light. Unlike liquid crystal displays (LCDs) that require backlighting, OLED displays are self-emissive devices—they emit light rather than modulate transmitted or reflected light. Accordingly, OLEDs are brighter, thinner, faster and lighter than LCDs, and use less power, offer higher contrast and are cheaper to manufacture.
An OLED display panel is driven by a driver including a row driver and a column driver. A row driver typically selects a row of OLEDs in the display panel, and the column driver provides driving current to one or more of the OLEDs in the selected row to light the selected OLEDs according to the display data.
Conventional OLED display panels have the shortcoming that cross-talk is generated in the display panel. The problem of cross-talk in conventional OLED display panels will be explained in greater detail below with reference to FIG. 1.
FIG. 1 illustrates a conventional OLED display panel driven by a conventional driver. The OLED display panel 100 comprises an array of OLEDs 102 coupled between the rows and columns of the display panel 100. The anodes of the OLEDs 102 are coupled to the columns and the cathodes of the OLEDs 102 are coupled to the rows of the display panel 100. The OLED display panel 100 is driven by driver including a row driver 120 and a column driver 140.
The row driver 120 includes row driver control circuitry (not shown) configured to couple the cathodes of the OLEDs associated with a row ( . . . ROW(n−1), ROW(n), ROW(n+1), ROW(n+2) . . . ) of the display panel 100 to either a low voltage (e.g., GND) via resistors ( . . . RL(n−1), RL(n), RL(n+1), RL(n) . . . ) by closing the switches 126 and opening the switches 124 to select the row or to a high voltage (e.g., VCC) by closing the switches 124 and opening the switches 126 to unselect the row. For example, in FIG. 1, ROW(n) is shown selected with the switch 126 associated with ROW(n) being closed to couple ROW(n) to GND. The selection of ROW(n) by the row driver 120 forward-biases the OLEDs 102 coupled to ROW(n).
The column driver 140 includes current sources 142 that provide current ( . . . I(n−1), I(n), I(n+1), and I(n+2) . . . ) to the columns (C(n−1), C(n), C(n+1), C(n+2) . . . ) of the panel 100 to drive OLEDs on the columns. Once a row is selected by the row driver 120, the current sources 142 of the column driver 140 generate current ( . . . I(n−1), I(n), I(n+1), and I(n+2) . . . ) for the corresponding columns (C(n−1), C(n), C(n+1), C(n+2) . . . ) according to the corresponding display data ( . . . Idata(n−1), Idata(n), Idata(n+1), Idata(n+2) . . . ) to drives the OLEDs 102 on the selected row. The amount of current ( . . . I(n−1), I(n), I(n+1), and I(n+2) . . . ) is typically generated to be multiples of a unit driving current (e.g., Iw) and proportional to the display data ( . . . Idata(n−1), Idata(n), Idata(n+1), Idata(n+2) . . . ).
In one embodiment, the display data may be 1-bit data indicating 2 levels of brightness, for example, bright (“1”) or dark (“0”), of the OLEDs 102. Thus, the current ( . . . I(n−1), I(n), I(n+1), I(n+2) . . . ) from the current sources 142 is generated to be, for example, 0 or Iw. In another embodiment, the display data may be 2-bit data indicating 4 levels of brightness, for example, very dark (“0”), dark (“1”), bright (“2), and very bright (“3”), of the OLEDs 102. Thus, the current ( . . . I(n−1), I(n), I(n+1), I(n+2) . . . ) from the current sources 142 is generated to be, for example, 0 or Iw, 2×Iw, or 3×Iw. The OLEDs 102 in the selected row (e.g., ROW(n)) are lit (Iw, 2×Iw, or 3×Iw) or unlit (zero current) based upon the current ( . . . I(n−1), I(n), I(n+1), and I(n+2) . . . ) corresponding to the columns (C(n−1), C(n), C(n+1), C(n+2) . . . ) of the panel 100.
As can be seen from FIG. 1, the sink current (Isink(n)) of a selected row (ROW(n)) is determined by the sum of the current ( . . . I(n−1), I(n), I(n+1), I(n+2) . . . ) driving the columns (C(n−1), C(n), C(n+1), C(n+2) . . . ) of the selected row (ROW(n)), which in turn is determined by the display data ( . . . Idata(n−1), Idata(n), Idata(n+1), Idata(n+2) . . . ). Therefore, the sink voltage Vsink(n) across RL(n) coupled to the selected row ROW(n) is also determined by the display data ( . . . Idata(n−1), Idata(n), Idata(n+1), Idata(n+2) . . . ), since Vsink(n)=Isink(n)×RL(n). This means that the sink voltage Vsink(n) for the rows of the panel 100 are different from each other, since the column display data varies from row to row. This will be explained in greater detail with reference to FIG. 2.
FIG. 2 is illustrates a sample image for display to a conventional OLED display panel 100 by the display data. As shown in FIG. 2, each of the columns 1-100 is driven by a unit current source Iw. The display data is configured to make the region 202 of the panel 100 “black” while making the remaining areas 204 “white.” Assuming a 2-bit display data (0 or 1), the current Iw will flow through the OLEDs coupled between row E and every column (0-100) to light the OLEDs on row E, making the-total sink current Isink(E) for row E as large as 100×Iw. In contrast, the current Iw will flow through the OLEDs coupled between row F and the columns 1-30 and columns 61-100 to light the OLEDs but not between row F and columns 31-60 on row F, making the total sink current Isink(F) for row F merely 70×Iw. Therefore, the sink voltages Vsink(E) and Vsink(F) on the resistors RL(E) and RL(F) coupled to rows E and F, respectively, will be: Vsink(E)=(Iw·100)·RL(E), and Vsink(F)=(Iw·70)·RL(F). Since RL(E) is equal to RL(F) in conventional row drivers, Vsink(E) becomes larger than Vsink(F), resulting in a forward-bias voltage for the OLEDs on Row F greater than the forward-bias voltage for the OLEDs on Row E.
FIG. 3 is a graph illustrating the driving voltage versus brightness characteristics of OLED pixels on a conventional OLED display panel 100. Line 302 illustrates the driving voltage versus brightness characteristics of the OLEDs on row E and line 304 illustrates the driving voltage versus brightness characteristics of the OLEDs on Row(F). As shown in FIG. 3, the OLEDs on Row F are brighter than the OLEDs on Row(E) for a given column driving voltage, because the cathodes of the OLEDs on Row(F) are biased with a voltage lower than-the voltage biasing the cathodes of the OLEDs on Row(E), i.e., the forward-bias voltage for the OLEDs on Row(F) is greater than the forward-bias voltage for the OLEDs on Row(E).
FIG. 4 illustrates a sample image that would be actually displayed on a conventional OLED display panel 100 by the display data due to differing forward-bias voltages for the OLEDs from row to row as illustrated in FIG. 3. Because the OLEDs on Row(F) are brighter than the OLEDs on Row(E), the regions 302 on Row(F) would display a “white” brighter than the “white” in regions 204 on Row (E). The difference in the brightness in these “white” regions 204, 304 is generally referred to as “crosstalk.”
Therefore, there is a need for a driver that can drive an OLED display panel without generating crosstalk.